Stars are discs, not points

[sevensisters1]

Hi all, I've just taken my first few images, but I keep getting the same problem... the stars appear as discs not sharp points:(  Here's an example:

I'm using the bulb setting and 20 sec exposure on a canon 1100D (with remote and tripod!), so the iso is set at 800... I think this was advised in another thread but doesn't seem to be working for me:(

Thanks in advance,

Sharon.

[Big_Daz]

Looks like a focus issue to me but I'm sure an astro photographer will be along soon to give you some good advise.

[nmoushon]

Thats definitely a focus problem. Looks like you are still out of focus. Putting the focus to infinity isn't going to get you good focus. It will get you close put not good enough. I know its hard with wide lenses. You just have to take sever test exposures and tweek it till you get it right. 

[wxsatuser]

Looks like focus to me as well.

It takes a bit of practice to focus and most lenses go past infinity.

Take some test shots and in playback you can zoom in to look at your results.

[Aenima]

The thing with it being 'wrong' to use infinity focus isn't so much that infinity is the wrong point to be at, but rather the infinity markings on lenses aren't always reliable enough for such a fine tuning as is needed for stars.

Try using zoom/liveview on a bright star to get it as close as possible to a pinpoint. Maybe even auto-focus on a very distant lamppost might help.

I f you can get something to visually look at while adjusting - try turning past the focus point (if it will go past) and pull back again and get a feel for the point where the stars are smallest and sharpest.

Ive seen bahtinov masks for dslr's but mine has yet to give me anything to work with.

Hope you get it sorted.

Regards

Aenima

[Leveye]

A focus issue for sure and a bit of trailing as well. If your camera has a focus assist zoom use that to focus on a bright star in the sky first. You're only going to get a few seconds at that focal length. I can only get about 25 seconds at 16mm with my wide angle lens on just a tripod no matter the ISO used. When you do nail the focus i can tell you're going to get great color keep at it!

[sgazer]

definitely focus. Use liveview on the 1100d and manual focus. Point to a reasonably bright star so liveview can see it, then zoom x10 on liveview and focus until it's the sharpest point possible. Then take a test exposure to check it's still sharp. Ideally, get  a bahtinov mask for your scope as this allows near perfect focusing, again using liveview. If you are using a camera lense, you can make a small one, there's a website which creates it and you can print it out.

ISO800 is generally a good balance between noise and brightness. If the object is too bright, eg orion nebula, then you might want lower ISO.

[chd]

I can often get decent results by focusing all the way out to the stop at infinity, and then bringing it back in just a hair. Different lenses vary though, so you may want to do some daylight tests on a high-contrast mountain ridge or something very far away so you get an idea of how much a "hair" is for your lens.

Even when you have good focus some brighter stars may appear as a disk, depending on exposure, because they can bloom, but dimmer ones should be points.

[m37]

You could knock up a hartmann mask out of cardboard. Disc with 3 triangles cut in it, when the triangles in the image converge you are in focus. Loads of templates on the web. Costs nowt.

Sent from my Nexus 7 using Tapatalk 4

[Astrosurf]

Melsky showed me to increase the shutter speed until a bright star dimmed to a point rather than a bloated disk on live view. You can control how bright the star is with that method. It works really well and improved my focusing no end. I hope you can do that with your camera!

[Okeydokey]

Very nice first few shots, and welcome!  Sure that doesn't help much... but I'm impressed

[sevensisters1]

Many thanks for all the replies, yes I'll keep tweaking the focus and check out one of those masks, practice makes perfect I suppose.. as someone said, patience is the most important accessory when it comes to AP

[m37]

Here's a page about it.

http://www.iceinspace.com.au/63-187-0-0-1-0.html

[Twisted Lip]

The only way I've ever managed to get focus is by using live view and zoom on the laptop, has anyone actually tried a Hartmann mask directly on the camera lens? I'm hopefully somewhere dark in a couple of weeks but will only have the camera (no laptop or telescope etc) so something like this would be a cracking help I think.

[Aenima]

The only way I've ever managed to get focus is by using live view and zoom on the laptop, has anyone actually tried a Hartmann mask directly on the camera lens? I'm hopefully somewhere dark in a couple of weeks but will only have the camera (no laptop or telescope etc) so something like this would be a cracking help I think.

I mention earlier in the thread about a bahtinov i bought for dslr lenses which I spent a frustrating night trying to find a combination of settings and bright star where i could even get a pattern, let alone successful focus - it might just be me, but it seemed that having it so close to the lens and the small amount of light from the star just wasn't effective.

The hartman mask i feel would be a better bet due to the shape being split then coming together when close to focus, they are also slightly easier to make at home i'd imagine...

My best focus has actually come from a very distant but bright streetlight or lampost using auto-focus and then clamping or switching off auto focus for imaging. But a dark site might be lacking in well, y'know.... lights.

Best of luck on it though.

Regards

Aenima 

[lensman57]

Hi all, I've just taken my first few images, but I keep getting the same problem... the stars appear as discs not sharp points:(  Here's an example:

I'm using the bulb setting and 20 sec exposure on a canon 1100D (with remote and tripod!), so the iso is set at 800... I think this was advised in another thread but doesn't seem to be working for me:(

Thanks in advance,

Sharon.

Hi,

Having been imaging now for just over 6 months I can you that you have focus issues there. Manual focusing and AP are not compatible and liveview focusing though much better, is still a hit and miss affair. You have two options, taking a lot of exposure of a bright star and altering the focus till you get close ( very time consuming ) but this will never be close enough if you ever decided to go serious about imaging, or use a laptop and software assisted focusing ( you will have to use this in the end anyway so get used to it from the start ). I use APT, fully functional even in the trial version and only £10.00 for the license  , and the FWHM focus aid. This is the most surefire way of getting things right and this is what everyone in the imaging section uses for DSOs and their stars are small and pinpiont even after hours of exposure. There are other reasons why even an in focus star would not appear as pinpoint  ( lens abberrations, atmospheric conditions, inefficient - filter coatings and so on ) but at this moment they are not relevant , one step at a time.

Regards,

A.G

PS: That is quite a nice capture, all the major stars are showing good and accurate colour so well done.

[Twisted Lip]

I mention earlier in the thread about a bahtinov i bought for dslr lenses which I spent a frustrating night trying to find a combination of settings and bright star where i could even get a pattern, let alone successful focus - it might just be me, but it seemed that having it so close to the lens and the small amount of light from the star just wasn't effective.

The hartman mask i feel would be a better bet due to the shape being split then coming together when close to focus, they are also slightly easier to make at home i'd imagine...

My best focus has actually come from a very distant but bright streetlight or lampost using auto-focus and then clamping or switching off auto focus for imaging. But a dark site might be lacking in well, y'know.... lights.

Best of luck on it though.

Regards

Aenima

Yes that was my worry with the BMask (being so close to the lens). I'm going to see if I can knock up a HMask this week and give it a go but as ever, it depends on the decent weather. If all else fails I may have to just drag the lappy out with me and see how it copes in the cold!

Cheers

Will

[ollypenrice]

You can't focus accurately by zooming in on star images. You have to measure them. There are astro capture programmes available for DSLRs, especially Cannons, which have Full Width Half Max tools for measuring the size of the stars as they are captured. You do need to do this. It might seem reasonable to guess at the smallest star size by looking at it while zoomed in but, while better than guessing, it simply isn't good enough.

I see this every night while focusing my CCD cameras. I have a zoomed in image of the star and I have the FWHM measurments coming in. As I refine the measured focus (and improve it drastically) there is no perceptible change to the stellar image on the screen.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233

[sevensisters1]

Many thanks for all the replies guys, what a great forum:) The other evening I just took a random early evening photo of the Aquila region and was surprised when the resulting stars were more point-like...

But later in the evening with the same settings, once more the stars had turned into ugly discs - next time I'll try shorter exposure times but will I capture the fainter stars in that case... I know I'm going to have to test out the more complex methods suggested above although they sound a bit daunting to a novice like me... o well must keep persevering:) Thanks again.

Sharon

[lensman57]

Many thanks for all the replies guys, what a great forum:) The other evening I just took a random early evening photo of the Aquila region and was surprised when the resulting stars were more point-like...

But later in the evening with the same settings, once more the stars had turned into ugly discs - next time I'll try shorter exposure times but will I capture the fainter stars in that case... I know I'm going to have to test out the more complex methods suggested above although they sound a bit daunting to a novice like me... o well must keep persevering:) Thanks again.

Sharon

Hi,

There is nothing daunting about using FWHM, it is as easy as reading a few numbers. You open the software, connect the camera to software, enable live view, pick a suitable star, no too dim or not too saturated. Do a visual focus while looking at the image of the star in the software window. Enable lthe FWHM focus aid, place  the cross or the little measuring window over the star and start reading the FWHM value, adjust focus untill the FWHM value is the smallest and the brighteness measure the highest It takes about 30~60s. A lot shorter and much more accurate than anything else. If you change your exposure or point to a different part of the sky do the same routine and your stars are always in "focus". Do yourself a favour and do it right in the first place please.

Regards,

A.G

[wxsatuser]

All part of the learning process.

[Alien 13]

Another useful tip if you are using the standard canon lenses which tend to have a sloppy focus ring is to apply blu-tak or a rubber band to it when you have achieved focus might be a bit fiddly but you have a better chance of holding focus during a session.

[Twisted Lip]

You can't focus accurately by zooming in on star images. You have to measure them. There are astro capture programmes available for DSLRs, especially Cannons, which have Full Width Half Max tools for measuring the size of the stars as they are captured. You do need to do this. It might seem reasonable to guess at the smallest star size by looking at it while zoomed in but, while better than guessing, it simply isn't good enough.

I see this every night while focusing my CCD cameras. I have a zoomed in image of the star and I have the FWHM measurments coming in. As I refine the measured focus (and improve it drastically) there is no perceptible change to the stellar image on the screen.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233

What do you find the easiest to use Olly? Any recommendations?

[ian_bird]

BackyardEOS is very good and cheap.

Cheers

Ian

[Alien 13]

BackyardEOS is very good and cheap.

Cheers

Ian

I think you get a 30day free trial with this too.